> I'm especially interested in fruit that might
>have grown in temperate regions.

There is one that comes to mind the pawpaw ( asimina triloba), that grow
wild in thickets from Canada border to North Florida the fruit is related to
cheromoya , an other one is prickly pear cactus fruit, cherries , the
actinindias family ( the kiwi fruit is part of that family but far from
being the tastiest, being a  domesticated  form), the wild persimmon,
etc...
there is a lot of fruits suitable to temperates climates which are still
wild somewhere on the globe. A good ressource to find lot of edibles or
usefull plants in tropical or temperates climates is : Cornucopia II,  by
stephen Facciola, kampong publications ( 700 pages of listings)
About pawpaw if somebody on that list enconter wild pawpaw trees while
walking in the wood i will be interested to get seeds  with little bit of
the soil where they grow ( a specific bacteria present in the soil  is
apparently  needed to good success in growing them)
 Are crab apples unchanged, for example?
There is lot of varietees of crabapples because they crosspollinate each
over and birds sow them , so the varieties are infinites. One that is truely
with a fixed genome is the pacific crab apple native from the west coast of
canada ,it grows wild here in the woods and give a fruit as big as  a big
blueberry. they are very sour before the frost and become very good after
frost.
jean-claude